Process for producing water-absorbing polymer particles

ABSTRACT

A process for producing water-absorbing polymer particles, wherein a polymer gel is removed from a polymerization reactor, stored intermediately as a particulate polymer gel in a delay vessel, removed by means of a first conveying device at the lower end of the delay vessel and dried, the intermediately stored particulate polymer gel being conveyed within the delay vessel by means of at least one second conveying device above the first conveying device in the direction of the first conveying device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 12/988,776,filed Oct. 20, 2010, now U.S. Pat. No. 8,329,844, which is the U.S.national phase application of International Application No.PCT/EP2009/054369, filed Apr. 14, 2009, which claims the benefit ofEuropean patent application No. 08155507.0, filed Apr. 30, 2008.

The present invention relates to a process for producing water-absorbingpolymer particles, wherein a polymer gel is removed from apolymerization reactor and stored intermediately in a delay vessel inthe form of particulate polymer gel.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

The polymer gel obtained by solution polymerization can be storedintermediately in an insulated vessel to increase the monomerconversion.

Water-absorbing polymers are used to produce diapers, tampons, sanitarynapkins and other hygiene articles, but also as water-retaining agentsin market gardening.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles.

The object is achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c) at least one initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers specified under a) and-   e) optionally one or more water-soluble polymers    in at least one polymerization reactor, the resulting polymer gel    being removed from the polymerization reactor, optionally    comminuted, stored intermediately in at least one delay vessel in    the form of particulate polymer gel, removed by means of at least    one first conveying device at the lower end of the delay vessel and    dried, wherein the intermediately stored particulate polymer gel is    conveyed within the delay vessel by means of at least one second    conveying device above the first conveying device in the direction    of the first conveying device.

When the polymer gel is not already obtained in the form of particulatepolymer gel in the polymerization reactor, the polymer gel has to becomminuted to a particulate polymer gel. Polymerization reactors inwhich a particulate polymer gel is obtained directly are, for example,kneading reactors. Polymerization reactors in which a particulatepolymer gel is not obtained are, for example, belt reactors.

The particle size of the particulate polymer gel is preferably from 0.1to 100 mm, more preferably from 0.5 to 10 mm, most preferably from 1 to3 mm.

The polymer gel is removed from the delay vessel by means of at leastone first conveying device, preferably by means of at least one screwconveyor, more preferably by means of at least one screw conveyor withtwo contrarotatory conveying screws, and preferably transferred by meansof a swivel belt into the drying device, for example a belt dryer.

When the first conveying device used is a screw conveyor, the speed ofthe conveying screw is preferably from 1 to 50 revolutions/minute, morepreferably from 5 to 30 revolutions/minute, most preferably from 10 to20 revolutions/minute. The external diameter of the conveying screw ispreferably from 0.05 to 0.5 m, more preferably from 0.1 to 0.4 m, mostpreferably from 0.2 to 0.3 m.

The polymer gel is conveyed within the delay vessel in the direction ofthe first conveying device by means of at least one second conveyingdevice, preferably by means of at least one screw conveyor, morepreferably by means of at least one screw conveyor with twocontrarotatory conveying screws, most preferably by means of two screwconveyors with two contrarotatory conveying screws each.

When the second conveying device used is a screw conveyor, the speed ofthe conveying screw is preferably from 0.05 to 5 revolutions/minute,more preferably from 0.1 to 2 revolutions/minute, most preferably from0.2 to 1 revolution/minute. The external diameter of the conveying screwis preferably from 0.1 to 1.5 m, more preferably from 0.25 to 1 m, mostpreferably from 0.5 to 0.75 m.

The conveying performance of the second conveying device should be lowerthan the conveying performance of the first conveying device in order toprevent undesired compaction of the polymer gel.

In a preferred embodiment of the present invention, screw conveyors witha combination of right-hand and left-hand threads are used as the secondconveying device. The combination of right-hand and left-hand threads onone shaft enables the transport of the particulate polymer gel from theouter regions to the center.

The conveying direction, i.e. the axes of rotation of the screwconveyors of the first conveying device and of the second conveyingdevice, are typically arranged offset horizontally. The angle enclosedis preferably from 45 to 90°, more preferably from 80 to 90°, mostpreferably from 85 to 90°.

The figures show preferred embodiments of the invention, the referencenumerals having the following meanings:

-   1 first conveying device-   2 second conveying device-   3 delay vessel-   4 particulate polymer gel

FIGS. 1 a and 1 b show two vertical sections, rotated by 90°, through adelay vessel with a screw conveyor as the first conveying device and ascrew conveyor as the second conveying device. The screw conveyor of thesecond conveying device has a right-hand and left-hand thread andconveys from the vessel wall to the center of the vessel.

FIGS. 2 a and 2 b show two vertical sections, rotated by 90°, through adelay vessel with a screw conveyor with two contrarotatory conveyingscrews as the first conveying device, and two screw conveyors with twocontrarotatory conveying screws each as the second conveying device. Thescrew conveyors of the second conveying device have a right-hand andleft-hand thread, and convey from the vessel wall to the center of thevessel.

The two screw conveyors according to FIGS. 2 a and 2 b areadvantageously arranged such that the conveying screws of the twoadjacent conveying screws just fail to intermesh. In contrast, the twocontrarotatory conveying screws of the screw conveyor in each casetypically intermesh.

Moreover, the size of the conveying screws should be selected such thatthe conveying screws of the second conveying device very substantiallyoccupy the horizontal cross section of the delay vessel, i.e. there areno areas in which the particulate polymer gel is not conveyed.

In the process according to the invention, it is possible to use delayvessels with a greater cross section than has been customary to date. Inspite of the increased cross section, there is no tendency to formbridges in the vessel. As a result, it is possible to implement delayvessels with a lower bed height. Owing to the lower bed height, theparticulate polymer gels are less highly compacted in the delay vesseland can be dried more easily.

The vertical distance between the first conveying device and the secondconveying device is preferably less than 0.5 m, more preferably lessthan 0.2 m, most preferably less than 0.1 m.

The residence time of the particulate polymer gel in the delay vessel ispreferably from 1 to 60 minutes, more preferably from 2 to 30 minutes,most preferably from 5 to 15 minutes.

The temperature of the particulate polymer gel in the delay vessel ispreferably from 60 to 100° C., more preferably from 70 to 95° C., mostpreferably from 80 to 90° C. Typically, the delay vessel is thermallyinsulated.

The bed height of the particulate polymer gel in the delay vessel ispreferably from 0.5 to 3 m, more preferably from 0.8 to 2.5 m, mostpreferably from 1 to 2 m.

The gas atmosphere in the delay vessel has a partial oxygen pressure ofpreferably less than 50 mbar, more preferably of less than 10 mbar, mostpreferably of less than 2 mbar. Oxygen scavenges free radicals which arepresent in the particulate polymer gel or form as a result of residualinitiator decomposition. However, these free radicals are needed toconvert the residual monomers.

The particulate polymer gel in the delay vessel has a water content ofpreferably from 30 to 70% by weight, more preferably from 35 to 65% byweight, most preferably from 40 to 60% by weight. Lower and higher watercontents lead to a reduced conversion of residual monomers in the delayvessel.

The water-absorbing polymer particles are produced by polymerizing amonomer solution or suspension and are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, acrylic acid purified according to WO2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203% by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, preferably at least 10 ppm by weight, more preferably at least30 ppm by weight, especially around 50 ppm by weight, of hydroquinonemonoether, based in each case on the unneutralized monomer a). Forexample, the monomer solution can be prepared by using an ethylenicallyunsaturated monomer bearing acid groups with an appropriate content ofhydroquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450A1, mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraalloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.05 to 1.5% by weight,more preferably from 0.1 to 1% by weight, most preferably from 0.3 to0.6% by weight, based in each case on monomer a). With risingcrosslinker content, the centrifuge retention capacity (CRC) falls andthe absorption under a pressure of 21.0 g/cm² (AUL0.3 psi) passesthrough a maximum.

The initiators c) may be all compounds which generate in free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators, photoinitiators. Suitable redox initiators are sodiumperoxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodiumperoxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite.Preference is given to using mixtures of thermal initiators and redoxinitiators, such as sodium peroxodisulfate/hydrogen peroxide/ascorbicacid. The reducing component used is, however, preferably a mixture ofthe sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium saltof 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixturesare obtainable as Brüggolite® FF6 and Brüggolite® FF7 (BrüggemannChemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Typically, an aqueous monomer solution is used. The water content of themonomer solution is preferably from 40 to 75% by weight, more preferablyfrom 45 to 70% by weight, most preferably from 50 to 65% by weight. Itis also possible to use monomer suspensions, i.e. oversaturated monomersolutions. With rising water content, the energy requirement in thesubsequent drying rises, and, with falling water content, the heat ofpolymerization can only be removed inadequately.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. The monomer solution can therefore be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingan inert gas through, preferably nitrogen or carbon dioxide. The oxygencontent of the monomer solution is preferably lowered before thepolymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors.In the kneader, the polymer gel formed in the polymerization of anaqueous monomer solution or suspension is comminuted continuously by,for example, contrarotatory stirrer shafts, as described in WO2001/038402 A1. Polymerization on a belt is described, for example, inDE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a beltreactor forms a polymer gel, which has to be comminuted in a furtherprocess step, for example in an extruder or kneader.

The acid groups of the resulting polymer gels have typically beenpartially neutralized. Neutralization is preferably carried out at themonomer stage. This is typically done by mixing in the neutralizingagent as an aqueous solution or preferably also as a solid. The degreeof neutralization is preferably from 25 to 95 mol %, more preferablyfrom 30 to 80 mol %, most preferably from 40 to 75 mol %, for which thecustomary neutralizing agents can be used, preferably alkali metalhydroxides, alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

However, it is also possible to carry out neutralization after thepolymerization, at the stage of the polymer gel formed in thepolymerization. It is also possible to neutralize up to 40 mol %,preferably from 10 to 30 mol % and more preferably from 15 to 25 mol %of the acid groups before the polymerization by adding a portion of theneutralizing agent actually to the monomer solution and setting thedesired final degree of neutralization only after the polymerization, atthe polymer gel stage. When the polymer gel is neutralized at leastpartly after the polymerization, the polymer gel is preferablycomminuted mechanically, for example by means of an extruder, in whichcase the neutralizing agent can be sprayed, sprinkled or poured on andthen carefully mixed in. To this end, the gel mass obtained can berepeatedly extruded for homogenization.

The polymer gel is then preferably dried with a belt dryer until theresidual moisture content is preferably from 0.5 to 15% by weight, morepreferably from 1 to 10% by weight, most preferably from 2 to 8% byweight, the residual moisture content being determined by the EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 230.2-05 “Moisture Content”. In the case of too high a residualmoisture content, the dried polymer gel has too low a glass transitiontemperature T_(g) and can be processed further only with difficulty. Inthe case of too low a residual moisture content, the dried polymer gelis too brittle and, in the subsequent comminution steps, undesirablylarge amounts of polymer particles with an excessively low particle sizeare obtained (“fines”). The solids content of the gel before the dryingis preferably from 25 to 90% by weight, more preferably from 35 to 70%by weight, most preferably from 40 to 60% by weight. Optionally, it is,however, also possible to use a fluidized bed dryer or a heatedplowshare mixer for the drying operation.

The polymer gel which has additionally been loosened in the gel bunkerby the process according to the invention can be dried better incomparison with polymer gel which has not additionally been pretreated.The process according to the invention is advantageously performedcontinuously.

Thereafter, the dried polymer gel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of the EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No. WSP220.2-05 “Particle Size Distribution”, where the proportions by mass ofthe screen fractions are plotted in cumulated form and the mean particlesize is determined graphically. The mean particle size here is the valueof the mesh size which gives rise to a cumulative 50% by weight.

The proportion of particles with a mean particle size of at least 150 μmis preferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the permeability(SFC). The proportion of excessively small polymer particles (“fines”)should therefore preferably be small.

The proportion of particles with a particle size of at most 850 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too great a particle size lower the swell rate.The proportion of excessively large polymer particles should thereforelikewise be small.

To further improve the properties, the polymer particles may bepostcrosslinked. Suitable postcrosslinkers are compounds which comprisegroups which can form covalent bonds with at least two carboxylategroups of the polymer particle. Suitable compounds are, for example,polyfunctional amines, polyfunctional amido amines, polyfunctionalepoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1,DE 35 23 617 A1 and EP D450 922 A2, or β-hydroxyalkylamides, asdescribed in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

Additionally described as suitable postcrosslinkers are cycliccarbonates in DE 40 20 780 C1,2-oxazolidone and its derivatives, such as2-hydroxyethyl-2-oxazolidone, in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1,2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/031482 A1.

Preferred postcrosslinkers are ethylene carbonate, ethylene glycoldiglycidyl ether, reaction products of polyamides with epichlorohydrinand mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred postcrosslinkers are2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

In addition, it is also possible to use postcrosslinkers which compriseadditional polymerizable ethylenically unsaturated groups, as describedin DE 37 13 601 A1.

The amount of postcrosslinker is preferably from 0.001 to 2% by weight,more preferably from 0.02 to 1% by weight, most preferably from 0.05 to0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the postcrosslinkersbefore, during or after the postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are chloride, bromide, sulfate, hydrogensulfate,carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate and lactate.Aluminum sulfate and aluminum lactate are preferred. Apart from metalsalts, it is also possible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, from 0.001 to 1.5%by weight, preferably from 0.005 to 1% by weight, more preferably from0.02 to 0.8% by weight, based in each case on the polymer particles.

The postcrosslinking is typically performed in such a way that asolution of the postcrosslinker is sprayed onto the dried polymerparticles. After the spraying, the polymer particles coated withpostcrosslinker are dried thermally, and the postcrosslinking reactioncan take place either before or during the drying.

The spraying of a solution of the postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers, plowshare mixers and paddle mixers. Particular preference isgiven to horizontal mixers such as plowshare mixers and paddle mixers,very particular preference to vertical mixers. Suitable mixers are, forexample, Lödige mixers, Bepex mixers, Nauta mixers, Processall mixersand Schugi mixers. However, it is also possible to spray on thepostcrosslinker solution in a fluidized bed.

The postcrosslinkers are typically used in the form of an aqueoussolution. The content of nonaqueous solvent or total amount of solventcan be used to adjust the penetration depth of the postcrosslinker intothe polymer particles.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting behavior and reduces thetendency to form lumps. However, preference is given to using solventmixtures, for example isopropanol/water, 1,3-propanediol/water andpropylene glycol/water, where the mixing ratio is preferably from 20:80to 40:60.

The thermal drying is preferably carried out in contact dryers, morepreferably paddle dryers, most preferably disk dryers. Suitable dryersare, for example, Bepex dryers and Nara dryers. Moreover, it is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream dryer, for examplea shelf dryer, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250° C.,preferably from 120 to 220° C., more preferably from 130 to 210° C.,most preferably from 150 to 200° C. The preferred residence time at thistemperature in the reaction mixer or dryer is preferably at least 10minutes, more preferably at least 20 minutes, most preferably at least30 minutes, and typically at most 60 minutes.

Subsequently, the postcrosslinked polymer can be classified again.

To further improve the properties, the postcrosslinked polymer particlescan be coated or subsequently moistened. Suitable coatings for improvingthe swell rate and the permeability (SFC) are, for example, inorganicinert substances, such as water-insoluble metal salts, organic polymers,cationic polymers and di- or polyvalent metal cations. Suitable coatingsfor dust binding are, for example, polyols. Suitable coatings forcounteracting the undesired caking tendency of the polymer particlesare, for example, fumed silica, such as Aerosil® 200, and surfactants,such as Span® 20.

The water-absorbing polymer particles produced by the process accordingto the invention have a moisture content of preferably from 0 to 15% byweight, more preferably from 0.2 to 10% by weight, most preferably from0.5 to 8% by weight, the water content being determined by the EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 230.2-05 “Moisture Content”.

The water-absorbing polymer particles produced by the process accordingto the invention have a centrifuge retention capacity (CRC) of typicallyat least 15 g/g, preferably at least 20 g/g, preferentially at least 22g/g, more preferably at least 24 g/g, most preferably at least 26 g/g.The centrifuge retention capacity (CRC) of the water-absorbing polymerparticles is typically less than 60 g/g. The centrifuge retentioncapacity (CRC) is determined by the EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 241.2-05“Centrifuge Retention Capacity”.

The water-absorbing polymer particles produced by the process accordingto the invention have an absorption under a pressure of 49.0 g/cm² (0.7psi) of typically at least 15 g/g, preferably at least 20 g/g,preferentially at least 22 g/g, more preferably at least 24 g/g, mostpreferably at least 26 g/g. The absorption under a pressure of 49.0g/cm² (AUL0.7 psi) of the water-absorbing polymer particles is typicallyless than 35 g/g. The absorption under a pressure of 49.0 g/cm² (AUL0.7psi) is determined analogously to the EDANA (European Disposables andNonwovens Association) recommended test method No. WSP 242.2-05“Absorption under pressure”, except that a pressure of 49.0 g/cm² (0.7psi) is established instead of a pressure of 21.0 g/cm² (0.3 psi).

The present invention further provides a vessel for performing theprocess according to the invention.

The present invention further provides an apparatus for producingwater-absorbing polymer particles, comprising at least onepolymerization reactor, at least one inventive vessel and at least onedrying apparatus.

Methods:

The measurements should, unless stated otherwise, be performed at anambient temperature of 23±2° C. and a relative atmospheric humidity of50±10%. The water-absorbing polymers are mixed thoroughly before themeasurement.

Residual Monomers

The content of residual monomers in the water-absorbing polymerparticles is determined by the EDANA (European Disposables and NonwovensAssociation) recommended test method No. 410.2-02 “Residual Monomers”.

The content of residual monomers in the polymer gels is determinedanalogously. In a departure from the above test method, for thispurpose, 1 g of polymer gel is stirred in 200 ml of 10% by weight sodiumchloride solution by means of a magnetic stirrer at 300revolutions/minute for a total of 6 hours. The stirrer bar used is adisk-shaped magnetic stirrer bar with a diameter of 30 mm and a heightof 12 mm.

The EDANA test methods are, for example, obtainable from the EuropeanDisposables and Nonwovens Association, Avenue Eugène Plasky 157, B-1030Brussels, Belgium.

EXAMPLE 1 Comparative

By continuously mixing water, 50% by weight sodium hydroxide solutionand acrylic acid, a 38.8% by weight acrylic acid/sodium acrylatesolution was prepared such that the degree of neutralization was 71.3mol %. The solids content of the monomer solution was 38.8% by weight.After mixing the components, the monomer solution was cooledcontinuously by means of a heat exchanger.

The polyethylenically unsaturated crosslinker used is polyethyleneglycol-400 diacrylate (diacrylate of a polyethylene glycol having a meanmolar mass of 400 g/mol). The amount used was 2 kg per t of monomersolution.

To initiate the free-radical polymerization, the following componentswere used: hydrogen peroxide (1.03 kg (0.25% by weight) per t of monomersolution), sodium peroxodisulfate (3.10 kg (15% by weight) per t ofmonomer solution) and ascorbic acid (1.05 kg (1% by weight) per t ofmonomer solution).

The throughput of the monomer solution was 17 t/h.

The individual components are metered continuously into a ListContikneter continuous kneader of capacity 6.3 m³ (from List, Arisdorf,Switzerland) in the following amounts:

17 t/h of monomer solution 34 kg/h of polyethylene glycol-400 diacrylate70.2 kg/h of hydrogen peroxide solution/sodium peroxodisulfate solution17.9 kg/h of ascorbic acid solution

Between the addition points for crosslinker and initiators, the monomersolution was innertized with nitrogen.

At the end of the reactor, 1000 kg/h of removed undersize with aparticle size less than 150 μm were additionally metered in.

At the feed, the reaction solution had a temperature of 23.5° C. Thereactor was operated with a shaft speed of 38 rpm. The residence time ofthe reaction mixture in the reactor was 15 minutes.

After polymerization and gel comminution, the aqueous polymer gel wasapplied via a delay vessel to a forced air belt dryer.

The delay vessel had a square base with an edge length of 2.2 m and aheight of 2.3 m.

The delay vessel had a screw conveyor with two contrarotatory conveyingscrews as the first conveying device and two screw conveyors with twocontrarotatory conveying screws each as the second conveying device. Thescrew conveyors of the second conveying device had a right-hand andleft-hand thread and conveyed from the vessel wall to the center of thevessel (FIGS. 2 a and 2 b).

The conveying screws of the first conveying device were operated at 14revolutions/minute, and the conveying screws of the second conveyingdevice were operated at 0.5 revolutions/minute.

The conveying screws of the first conveying device had an externaldiameter of 250 mm. The distance between the shafts of thecontrarotatory conveying screws was 180 mm.

The conveying screws of the second conveying device had an externaldiameter of 630 mm. The distance between the shafts of thecontrarotatory conveying screws was 465 mm.

The delay vessel was operated such that no fill level could build up.The second conveying device was switched off.

The dried hydrogel had a residual moisture content of approx. 3% byweight and was coarsely comminuted.

The content of residual monomers (remos) was determined before and afterthe drying, as was the proportion of polymer particles having a particlesize of greater than 10 mm after the coarse comminution (agglomerates).The results are summarized in the tables.

EXAMPLE 2 Comparative

The procedure was as in example 1. The fill level in the delay vesselwas between 60 and 80%. The second conveying device remained switchedoff.

EXAMPLE 3

The procedure was as in example 1. The fill level in the delay vesselwas between 60 and 80%. The second conveying device was switched on.

TABLE Results Second Remos Remos Fill conveying before after Ex. leveldevice drying Agglomerates drying 1* None Switched off 0.8% by wt. <0.5t/h 600 ppm 2* 60-80% Switched off 0.3% by wt.  1.2 t/h 320 ppm 3 60-80%Switched on 0.3% by wt. <0.5 t/h 310 ppm *Comparative example

The invention claimed is:
 1. A vessel for storing a particulate polymer gel comprising at least one rotating first conveying device within and at a lower end of the vessel and at least one rotating second conveying device within the vessel and above the first conveying device, wherein the at least one rotating second conveying device conveys in a direction toward the at least one rotating first conveying device, and an axis of rotation of the at least one rotating first conveying device is offset horizontally from an axis of rotation of the at least one rotating second conveyor device.
 2. The vessel according to claim 1, wherein a vertical distance between the first conveying device and the second conveying device is less than 0.5 m.
 3. The vessel according to claim 1, wherein the first conveying device is a screw conveyor.
 4. The vessel according to claim 1, wherein the second conveying device is a screw conveyor.
 5. The vessel according to claim 1, wherein the second conveying device is a screw conveyor with a combination of right-hand and left-hand thread.
 6. The vessel according to claim 1, wherein the first conveying device and/or the second conveying device is at least one screw conveyor with two contrarotatory conveying screws.
 7. An apparatus for producing water-absorbing polymer particles comprising at least one polymerization reactor, at least one vessel according to claim 1, and at least one drying apparatus.
 8. The vessel according to claim 1 wherein a conveying performance of the second conveying device is lower than a conveying performance of the first conveying device.
 9. The vessel according to claim 1 wherein the axis of rotation of the at least one rotating first conveying device is offset horizontally from the axis of rotation of the at least one rotating second conveying device by from 45° to 90°.
 10. The vessel according to claim 1 wherein the axis of rotation of the at least one rotating first conveying device is offset horizontally from the axis of rotation of the at least one rotating second conveying device by from 80° to 90°.
 11. The vessel according to claim 1 wherein the axis of rotation of the at least one rotating first conveying device is offset horizontally from the axis of rotation of the at least one rotating second conveying device by from 85° to 90°. 